Load-handling apparatus with boom-mounted constant attitude load-handling means

ABSTRACT

A mobile loader of the type having an articulated boom structure pivoted to a mobile base and with a log grapple mechanism pivoted to the free end of the boom structure. The boom structure includes an inner main boom pivoted to the base for movement in a vertical plane and an outer boom stick pivoted to the outer end of the main boom for movement in the same vertical plane. A heel boom is pivoted to the outer end of the boom stick and includes a special mounting hub incorporating a sprocket which pivots with the heel boom about its pivot axis. The sprocket forms part of a mechanical linkage differential system which extends within the boom structure to the base and which maintains the heel boom in a constant attitude with respect to the base during vertical pivoting movement of the boom structure. This differential system includes an idler sprocket at the intersection of the main boom and the boom stick, and a rod, lever and cylinder linkage connecting such sprocket to the base in a manner permitting actuation of the heel boom independently of pivotal movement of the main boom and boom stick.

United States Patent [72] lnventor Lynn F. Perrott 15928 N.E. Rose Parkway, Portland, Oreg. 97230 211 Appl. No. 808,090 [22] Filed Mar. 18,1969 [45] Patented July 6,1971

[54] LOAD-HANDLING APPARATUS WITH BOOM- MOUNTED CONSTANT ATTITUDE LOAD- HANDLING MEANS 18 Claims, 7 Drawing Figs.

[52] US. Cl. 214/147, 214/772 [51] Int. Cl 1366c 1/42 [50] Field 01 Search 214/147 AS, 147, 772,147 G [56] References Cited UNITED STATES PATENTS 2,457,049 12/1948 Lacey 214/772 3,182,813 5/1965 Goodell... 214/147 (G) X 3,204,795 9/1965 Larson 214/147 (AS) 3,362,549 1/1968 Ewart 214/147 (AS) Primary Examiner-Gerald M. Forlenza Assistant Examiner-George F. Abraham Atrorney- Buckhorn, Blore, Klarquist and Sparkman ABSTRACT: A mobile loader of the type having an articulated boom structure pivoted to a mobile base and with a log grapple mechanism pivoted to the free end of the boom structure. The boom structure includes an inner main boom pivoted to the base for movement in a vertical plane and an outer boom stick pivoted to the outer end of the main boom for movement in the same vertical plane. A heel boom is pivoted to the outer end of the boom stick and includes a special mounting hub incorporating a sprocket which pivots with the heel boom about its pivot axis. The sprocket forms part of a mechanical linkage differential system which extends within the boom structure to the base and which maintains the heel boom in a constant attitude with respect to the base during vertical pivoting movement of the boom structure. This differential system includes an idler sprocket at the intersection of the main boom and the boom stick, and a rod, lever and cylinder linkage connecting such sprocket to the base in a manner permitting actuation of the heel boom independently of pivotal movement of the main boom and boom stick.

PATENTEB JUL 6 I971 SHEET 1 OF 2 BUCK/405w, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS LOAD-HANDLING APPARATUS WITH BOOM-MOUNTED CONSTANT ATTITUDE LOAD-HANDLING MEANS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to load-handling apparatus of the type having a boom-mounted load-handling device and more particularly to a differential system for maintaining the load-handling device or a portion thereof in a constant attitude with respect to a reference plane during pivoting movement of the boom. The invention has particular application to mobile log and other load-lifting and loading equipment of the type having a grapple and a heel boom or plate carried at the free end of the main boom structure for maintaining the heel boom in a constant attitude with respect to a mounting base during vertical pivoting movement of the main boom.

2. Description of the Prior Art Loading equipment of the type having a boom-mounted grapple for lifting logs and other long loads commonly incorporates a sofcalled heel boom against which the load heels when lifted by the grapple at an offcenter point of the load so as to stabilize the same. During vertical swinging movement of the main boom structure, the attitude of the load relative to the ground surface will ordinarily change with the attitude of the main boom, requiring the use of adjustable heel booms, grapples or both and requiring constant readjustment of the grapple, heel boom or an outer portion of the main boom by the operator as he swings the main boom in order to maintain the load in a desirable attitude for loading. Naturally, this continual readjustment of the attitude of the load during the loading operation requires the constant attention of the operator and therefore slows down the loading operation and greatly increases its difficulty. I

SUMMARY OF THE INVENTION The present invention overcomes the problems of prior art heavy-duty load-handling equipment by providing mechanical differential system which can be carried within an articulated boom structure and which will automatically adjust the attitude of a heel boom or other load-handling element relative to the boom structure during pivoting movement of the latter so that the element maintains a constant attitude relative to the ground surface, base of the equipment or other reference plane during the load-handling operation without requiring the attention of the operator. Moreover, the differential system is capable of adjusting the attitude of the loadhandling element independently of the angular disposition and movement of the boom structure and does not increase appreciably the weight supported by the main boom structure. The constant attitude features of the invention are particularly suited for use with log loaders and other heavy-duty boom-mounted material-handling equipment such as cranes and excavators.

The primary objects of the invention are to provide:

1. load-handling apparatus having a boom-mounted loadhandling element which can be maintained automatically in a constant attitude relative to a reference plane during pivoting movement of the boom;

2. a boom-mounted differential system incorporating mechanical linkage means for maintaining a boom-mounted load-handling element in a constant attitude relative to a base plane as the attitude of its supporting boom changes;

3. a differential system as aforesaid which can readily be integrated with a boom structure without increasing appreciably the weight of such structure;

4. a differential system as aforesaid capable of use with equipment for handling exceptionally heavy loads,

5. a load-handling apparatus having an articulated boom structure and a heel boom which can be maintained automatically in a constant attitude relative to a reference plane as the attitude of the boom structure or portions thereof are changed relative to the same plane;

6. a log loader having a dead" log grapple pivoted to the free end of a boom structure and a live" heel boom mounted at the same end of the boom structure, wherein the heel boom can be maintained in a constant attitude as the attitude of the main boom changes relative to a base plane so that logs lifted by the grapple can also be maintained in a predetermined attitude as the attitude of the main boom changes;

7. constant attitude load-handling apparatus as aforesaid which is economical to manufacture and simple and economical to operate and maintain; and

8. load-handling apparatus as aforesaid incorporating a load-handling element with a constant attitude feature wherein the attitude of the element may be selectively changed.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects and advantages of the present invention will become more apparent from the following detailed description which proceeds with reference to the following drawings wherein:

FIG. I is a side elevational view of a log loader incorporating the present invention, illustrating in phantom lines various positions of adjustment of its boom structure;

FIG. 2 is an enlarged fragmentary elevational view, partly in section, showing the boom structure of the loader of FIG. 1;

FIG. 3 is a vertical sectional view taken along the line 3-3 of FIG. 2 on a greatly enlarged scale;

FIG. 4 is a sectional view taken along the line 4-4 of FIG. 2 on approximately the same scale as FIG. 2;

FIG. 5 is a sectional view taken along the line 5-5 of FIG. 2 on approximately the same scale as FIG. 4;

FIG. 6 is a foreshortened side view on a greatly enlarged scale showing the details of the chain and rod connection of FIG. 2; and

FIG. 7 is a fragmentary view of an outer end portion of a boom structure incorporating a modified form of heel boom.

DETAILED DESCRIPTION With reference to the drawings, FIG. 1 discloses a log loader including a mobile main frame 10 supporting a driver's cab 12 and a turntable 13 which in turn supports a base member or platform 14. The frame is supported on wheels 16. The base member supports an operator's compartment 18 and mounts near its rear end an articulated boom structure indicated generally at 20. An internal combustion engine 22 on the base member drives a pump (not shown) which supplies pressure fluid to various hydraulic power cylinders used in pivoting the boom structure in a generally vertical plane.

Turntable 13 is powered by engine 22 and swings the base member and thus the boom structure about a vertical axis so that the boom structure can be extended to opposite sides and forwardly of the frame as well as to the rear thereof.

Main boom 20 includes an inner main boom 26 pivoted at its lower end by a boom-mounting means 28 on a pedestal portion 30 of the base for pivoting movement about a first horizontal pivot axis 32 (FIG. 3) of a fixed pivot shaft 34 forming part of the boom-mounting means. The boom also includes an outer boom stick 36 connected near its inner end by a fixed pivot shaft 38 to the outer end of main boom 26 for articulation in a vertical plane.

An inner end extension 40 of the boom stick is connected by a hydraulic cylinder 42 to an intermediate portion of boom 26 so as to provide for articulation of the boom stick relative to the main boom about the axis 44 of pivot shaft 38. The main boom is pivoted on base pivot shaft 34 by a second hydraulic cylinder 46 connected at 48 to an intermediate portion of the main boom section and at 50 to the base member.

The outer end of boom stick 36 carries a log-grappling mechanism 52 of any one of several well-known types. Such grapple mechanism usually includes a laterally opposed pair of tong members 54, only one of which is shown, which can be actuated toward and away from one another by fluid power cylinders 56 extending between the tongs and a connecting member 58. Member 58 is carried by a swivel member 60 which in turn is pivoted to the free end of the boom stick 61. A suitable grapple is shown in greater detail, for example, in U.S. Pat. No. 3,204,795 issued Sept. 7, 1965, in the name of R. W. Larson. Although a grapple is shown, it is to be understood that any other suitable load-lifting means may be employed at the other end of the boom depending on the use to which the load-handling apparatus is to be put, such as, for example, any sort of hook, sling, tong or bucket. The grapple shown is a socalled dead grapple-type because its attitude cannot be adjusted relative to the main boom. However, a live" grapple, adjustable relative to the main boom, could also be used if desired.

The load-handling means also includes a heel boom 64 which is pivoted at one end by a mounting means to the outer end of the boom stick at the same point 61 at which grapple 52 is attached. The heel boom extends generally downwardly and rearwardly from its connection with the boom stick. As shown in FIG. 5, the heel boom and grapple mounting means includes a pivot shaft 66 extending through and rotatably supported by hub portions 67, 68 on opposite sideplates 69, 70 of boom stick 36. Pivot shaft 66 is supported within hubs 67, 68 by bushings 72 which permit relative rotation between the shaft and the boom stick, A sprocket 74 forming part of a mechanical differential motion-transmitting means is keyed to a central portion of shaft 66 so as to pivot with the shaft. Also keyed to outer portions of shaft 66 to pivot with the shaft are opposite sideplates 76, 77 of heel boom 64. Mounting portions 78, 79 suspend the log grapple 52 on bushings 80 from shaft 66. From the foregoing it will be apparent that the heel boom and sprocket 74 can pivot together on shaft 66 about pivot axis 82 of such shaft and relative to boom stick 36.

Referring to the pivotal connection between the main boom and boom stick as shown best in FIG. 4, pivot shaft 38 extends through hub portions 81, 83 on sideplates 84, 85 of main boom section 26. Shaft 38 is fixed against rotation relative to the main boom by its noncircular enlarged head portion 86 seated within a like-shaped recess of a plate 87 welded to sideplate 85 of boom 26. The opposite, threaded end'of shaft 38 is secured by a nut 88. Sideplates 69, 70 of boom stick 36 have hub portions 90, 91 supported by bushings 92 for pivotal movement on shaft 38 and thus relative to main boom 26. Shaft 38 mounts a second, idler sprocket 94 on bushing 95 for rotation about axis 44 of the shaft independently of articulation between the main boom and boom stick. An endless chain means 96 is trained about idler sprocket 94 and heel boom sprocket 74 whereby pivotal movement of sprocket 94 on shaft 38 will effect a corresponding pivotal movement of heel boom sprocket 74 and thus the heel boom with shaft 66 about its pivot axis 82. For this purpose, the two sprockets have equal diameters. The two sprockets and chain also serve to restrain the heel boom against angular movement relative to the ground or base despite vertical swinging movement of the boom or boom stick through restraint of idler sprocket 94 by means about to be described. The sprockets and endless chain means 96 thus comprise part of the differential motion-transmitting means which extends along and mostly within the boom and boom stick from the load-handling means or heel boom to the base.

An additional portion of the differential means includes rod means extending from idler sprocket 94 to the base of the machine. Referring to FIGS. 2 and 4, this rod means includes a connecting rod 98 pivoted at 100 to idler sprocket 94 and pivoted at 103 to the upper end of a lever 102 intermediate the opposite ends of boom 26. Lever 102 is pivoted at 104 to a mounting plate portion 105 of the main boom. The rod means also includes an extensible fluid-powered cylinder 108 including a piston rod 109 pivoted at 110 to the lower end of lever 102 and pivoted at 112 to a pair of anchoring ears 114 rigidly fixed base pedestal 30. The pivotal connection 112 of cylinder 108 to anchor ears 114 is offset from pivot axis 32 of boom pivot shaft 34 to the same extent and in the same direction as the pivotal connection is offset from the pivot axis 44ofv idler sprocket 94. As a result, any angular movement of boom 26 about base pivot axis 32 in a given direction will effect a relative angular movement between idler sprocket 94 and boom 26 in a subtractive direction to an extent such that pivot point 100 remains in a fixed position relative to the axis 44 of shaft 38 regardless of the angulation of boom 26 relative to its base.

Expressed somewhat differently, the differential system shown will retain idler sprocket 94 and thus heel boom sprocket 74 and heel boom 64 in constant angular positions relative to the ground or base of the machine during pivotal movement of boom 26 under the influence of cylinder 46. When boom stick cylinder 42 is extended and retracted to pivot boom stick 36 about pivot axis 44 relative to boom 26, the differential linkage between the idler sprocket 94 and anchor 114 will prevent idler sprocket 94 from rotating on shaft 38, thereby maintaining heel boom sprocket 74 and the heel boom itself in a constant attitude relative to the ground.

With reference to the boom-mounting means 28 shown in detail in FIG. 3, pivot shaft 34 extends through and is supported by four upright supports 116, 117, 118, 119 of base pedestal 30. A pair of trunnions 121, 122 forming the base of boom 26 are mounted by bushings 124, 125 on pivot shaft 34 between base supports 116, 117 and 118, 119 respectively to provide pivoting movement of the main boom section in a vertical plane. The lower end of cylinder 108 is pivoted on pin 112 between anchor ears 114 so as to be capable of pivoting movement about the axis of pin '112 which is offset as previously indicated from pivot axis 32 of the main boom. Cylinder 108 is centered with respect to anchor ears 114 by spacer sleeves 127.

It will be apparent from FIG. 2 that most of the differential means heretofore described, including the endless chain means 96, idler sprocket 94, connecting rod 98, lever 102 and cylinder 108 extend along and within the outer shielding or wrap of main boom 26 and boom stick 36 for protection and for preventing possible fouling with the load.

The upper flight of endless chain 96, after appreciable use, would naturally tend to stretch and therefore develop slack, particularly if the chain were constructed entirely of chain links. For this reason and because of the limited movement through which the chain is required to travel, the upper flight of the chain means is made up of a combination of a chain-link section 128 which extends about the sprockets, and a length of steel rod 130 extending between the opposite ends of chain link section 128 along the straight portion of the boom stick between sprockets, as shown best in FIGS. 2 and 6. Chain section 128 also constitutes the lower flight of the chain means. Slack adjusters 132, 133 interconnect the opposite ends of rod 130 and chain section 128. The slack adjusters are cylindrical in form and have a threaded opening 134 in one end which receives a threaded end portion 135 of rod 130. The opposite end of each slack adjuster is pivoted by a pin 136 to the adjacent end of connecting chain segment 128. If slack should develop in the chain, the proper tension may be restored simply by threading the rod ends further into the slack adjusters. For this purpose one end of the rod 130 is threaded with right-hand threads while the opposite end is provided with left-hand threads, whereby both ends may be threaded simultaneously into their corresponding slack adjusters by rotation of the rod length in one direction.

As shown in FIG. 7, grapple 52 could be mounted on a forward extension 162 of a heel boom 164, rather than freely at the end of boom stick 36. This would provide the load with a constant attitude during pivoting movement of the boom structure while providing a live grapple as well as live heel boom.

The differential system disclosed is also applicable to any boom-mounted loadhandling means for which it is desired to maintain a constant attitude during swinging movement of the boom, such as, for example, an excavating bucket or a boommounted cage commonly used by public utilities and the fruitharvesting industries.

. counterclockwise direction which OPERATION Referring especially to FIG. 1, the differential system disclosed operates as follows: The desired attitude of heel boom 64 relative to base 14, the surrounding terrain or the grapple 52 may be determined independently of the position of the boom structure through extension or retraction of cylinder 108 forming part of the mechanical differential linkage. For example, with reference to FIG. 2, extension of cylinder 108 in the position of boom structure shown pivots lever 102 in a in turn rotates idler sprocket 94 in a counterclockwise direction. Rotation of idler sprocket 94 effects a corresponding counterclockwise rotation of heel boom sprocket 74 through an are determined by the degree of extension of cylinder 108. Rotation of heel boom sprocket 74 through a predetermined arc also swings heel boom 64 in the same rotational direction through the same arc, such motion being transmitted through the heel boom pivot shaft 66. This feature enables the attitude of heel boom 64 to be determined independently of the position of either main boom 26 or boom stick 36.

After the desired attitude of the heel boom relative to the ground has been determined, it may be maintained in such position during the loading operation and during the vertical pivoting movement of the main boom and boom stick merely by maintaining hydraulic cylinder 108 at its preset fixed length. This maintains the idler and heel boom sprockets in their preset angular positions relative to the base during the articulation of the boom structure.

Thus, for example, during vertical swinging movement of boom stick 36 from its solid line position A in FIG. 1 upwardly to its elevated dashed line position B through the arc AB in a counterclockwise direction about pivot pin 38, idler sprocket 94 is held in its preset position on shaft 38. This causes a rotation of sprocket 74, shaft 66 and heel boom 64 about the axis of shaft 66 through an are equal to the arc A-B but in a subtractive or clockwise direction to maintain the heel boom sprocket and thus the heel boom in their present attitudes relative to base 14 despite the changed attitude of the boom stick relative to such base. Therefore a log L supported by grapple 52 and stabilized by heel boom 64 in a substantially horizontal position will remain in that position despite the changing attitude of the boom stick.

If, after the boom stick is moved from its initial position A to its upper position B as shown in FIG. 1, the main boom is pivoted downwardly in a clockwise direction through an arc A-C to a lowered position C while the boom stick remains in its same position B relative to the main boom, the differential system causes idler sprocket 94 and the interconnected heel boom sprocket to pivot about their respective pivot axes through equal arcs in a subtractive or counterclockwise direction to maintain the sprockets, and thus heel boom 64, in their preset rotational attitudes relative to base 14. ln other words, as the main boom pivots in a clockwise direction on base pivot shaft 34 through the arc AC, heel boom 64 pivots about its pivot axis 82 in an opposite direction through an equal are so that the attitude of the heel boom relative to the ground remains constant while its attitude relative to the boom structure changes. Thus the log L illustrated can be maintained in a constant predetermined attitude relative to the ground, such as the horizontal attitude illustrated, from the time it is picked up by grapple 52 to the time it is unloaded, through all manners of articulation of the main boom and heel boom relative to each other and relative to the base.

Having illustrated a preferred embodiment of my invention, it should be apparent to those having skill in the art that the same permits of modification in arrangement and detail. I claim as my invention all such modifications as come within the true spirit and scope of the following claims.

1. A load-handling apparatus comprising:

a base,

a boom pivoted to said base for vertical movement about a first pivot axis,

load-handling means pivoted to said boom for vertical movement about a second pivot axis spaced along said boom from said first pivot axis,

and mechanical differential motion-producing means extending along said boom and operatively interconnecting said load-handling means and said base in a manner so that pivotal movement of said boom about said first pivot axis in one direction effects a pivotal movement of said load-handling means relative to said boom about said second pivot axis in a subtractive direction through an are sufficient to maintain said load-handling means in a constant attitude relative to said base,

said differential means including:

means mounted for pivotal movement about said second pivot axis with said load-handling means and including a portion offset from said second pivot axis, mechanical linkage means extending from said offset portion within said boom to anchor means fixedly connected to said base, said linkage means being connected to said anchor means at a position offset from said first pivot axis.

2. Apparatus according to claim 1 wherein said differential means includes attitude-determining means for selectively changing the attitude of said load-handling means relative to said base independently of movement of said boom.

3. Apparatus according to claim 2 wherein said attitudedetermining means includes extensible hydraulic cylinder means operatively connected to said load-handling means.

4. Apparatus according to claim 1 wherein the distance between said first pivot axis and the connection of said link means to said anchor means is the same as the distance between said second pivot axis and the connection between said linkage means and said offset portion.

5. Apparatus according to claim 1 wherein said linkage means includes an extensible hydraulic cylinder between said anchor means and said offset portion. I

6. Apparatus according to claim 1 wherein said boom includes a main boom pivoted to said base and a boom stick pivoted to said main boom for vertical movement about a third pivot axis, said load-handling means being pivoted to said boom stick.

7. Apparatus according to claim 6 wherein said differential means includes first sprocket means mounted for pivotal movement with said load-handling means about said second pivot axis, second sprocket means mounted for pivotal movement about said third pivot axis, endless chain means interconnecting said first and second sprocket means, and connecting rod means interconnecting said second sprocket means and said base.

8. Apparatus according to claim 7 wherein said rod means includes extensible cylinder means for selectively changing the attitude of said load-handling means relative to said base independently of movement of said main boom and said boom stick.

9. Apparatus according to claim 7 wherein said rod means includes lever means fixed to said main boom intermediate the opposite ends thereof, extensible cylinder means extending between and operatively connected to said lever means and either one of said second sprocket means or said base, and a connecting rod operatively connected to said lever means and the other of said second sprocket means or said base.

10. Apparatus according to claim 7 wherein said chain means includes a chain section trained about said first and second sprocket means and a rod section extending along said boom between said sprocket means on the tension side of said chain means, and chain-tensioning means interconnecting the ends of said chain section and the opposite ends of said rod section.

11. Apparatus according to claim 10 wherein said rod section is threaded at its opposite ends and said chain-tensioning means includes female threaded members threaded into engagement with the threaded ends of said rod section and pivotally connected to corresponding ends of said chain section.

12. A load-handling apparatus comprising:.

a boom,

a base,

boom-mounting means mounting said boom on said base for pivotal movement in a generally vertical plane about a first pivot axis,

load-handling means,

load-handler mounting means mounting said load-handling means on said boom-at a position spaced from said first pivot axis for pivotal movement of said load-handling means about a second pivot axis,

and mechanical differential motion-transmitting means extending along said boom and operatively connected at one end to said base and at the opposite end to said loadhandler mounting means,

said differential means including anchoring means in a fixed position relative to said base and means connected to said load-handler mounting means for pivotal movement with said load-handling means about said second pivot axis,

said anchoring means anchoring said differential means at a fixed position to said base, said position being offset from said first pivot axis so that as said boom is pivoted about said first pivot axis in one direction, said differential means effects a subtractive differential movement of said load-handling means relative to said boom about said second pivot axis through an arc sufficient to maintain said load-handling means in a constant attitude relative to said base,

said differential means including attitude-determining means for selectively actuating said differential means independently of pivotal movement of said boom for changing the attitude of said load-handling means relative to said base.

13. Apparatus according to claim 1 wherein said boom includes at least two boom sections pivotal relative to one another about a third pivot axis and wherein said differential means includes idler means pivotable about said third pivot axis.

14. Apparatus according to claim 1 wherein said differential means extends within said boom.

15. Apparatus according to claim 1 wherein said load-handling apparatus comprises a log loader with said base comprising a mobile base, a log-lifting means pivoted to the free end of said boom, said load-handling means comprising a heel boom pivoted to the free end of said boom.

16. Apparatus according to claim ll including load-lifting means carried near the free end of said boom for cooperation with said load-handling means.

17. Apparatus according to claim 16 wherein said load-handling means comprises a heel boom and said load-lifting means is pivoted to said boom for movement about said second pivot axis.

18. Apparatus according to claim 16 wherein said load-handling means comprises a heel boom and said load-lifting means is pivoted to said heel boom. 

1. A load-handling apparatus comprising: a base, a boom pivoted to said base for vertical movement about a first pivot axis, load-handling means pivoted to said boom for vertical movement about a second pivot axis spaced along said boom from said first pivot axis, and mechanical differential motion-producing means extending along said boom and operatively interconnecting said loadhandling means and said base in a manner so that pivotal movement of said boom about said first pivot axis in one direction effects a pivotal movement of said load-handling means relative to said boom about said second pivot axis in a subtractive direction through an arc sufficient to maintain said load-handling means in a constant attitude relative to said base, said differential means including: means mounted for pivotal movement about said second pivot axis with said load-handling means and including a portion offset from said second pivot axis, mechanical linkage means extending from said offset portion within said boom to anchor means fixedly connected to said base, said linkage means being connected to said anchor means at a position offset from said first pivot axis.
 2. Apparatus according to claim 1 wherein said differential means includes attitude-determining means for selectively changing the attitude of said load-handling means relative to said base independently of movement of said boom.
 3. Apparatus according to claim 2 wherein said attitude-determining means includes extensible hydraulic cylinder means operatively connected to said load-handling means.
 4. Apparatus according to claim 1 wherein the distance between said first pivot axis and the connection of said link means to said anchor means is the same as the distance between said second pivot axis and the connection between said linkage means and said offset portion.
 5. Apparatus according to claim 1 wherein said linKage means includes an extensible hydraulic cylinder between said anchor means and said offset portion.
 6. Apparatus according to claim 1 wherein said boom includes a main boom pivoted to said base and a boom stick pivoted to said main boom for vertical movement about a third pivot axis, said load-handling means being pivoted to said boom stick.
 7. Apparatus according to claim 6 wherein said differential means includes first sprocket means mounted for pivotal movement with said load-handling means about said second pivot axis, second sprocket means mounted for pivotal movement about said third pivot axis, endless chain means interconnecting said first and second sprocket means, and connecting rod means interconnecting said second sprocket means and said base.
 8. Apparatus according to claim 7 wherein said rod means includes extensible cylinder means for selectively changing the attitude of said load-handling means relative to said base independently of movement of said main boom and said boom stick.
 9. Apparatus according to claim 7 wherein said rod means includes lever means fixed to said main boom intermediate the opposite ends thereof, extensible cylinder means extending between and operatively connected to said lever means and either one of said second sprocket means or said base, and a connecting rod operatively connected to said lever means and the other of said second sprocket means or said base.
 10. Apparatus according to claim 7 wherein said chain means includes a chain section trained about said first and second sprocket means and a rod section extending along said boom between said sprocket means on the tension side of said chain means, and chain-tensioning means interconnecting the ends of said chain section and the opposite ends of said rod section.
 11. Apparatus according to claim 10 wherein said rod section is threaded at its opposite ends and said chain-tensioning means includes female threaded members threaded into engagement with the threaded ends of said rod section and pivotally connected to corresponding ends of said chain section.
 12. A load-handling apparatus comprising: a boom, a base, boom-mounting means mounting said boom on said base for pivotal movement in a generally vertical plane about a first pivot axis, load-handling means, load-handler mounting means mounting said load-handling means on said boom at a position spaced from said first pivot axis for pivotal movement of said load-handling means about a second pivot axis, and mechanical differential motion-transmitting means extending along said boom and operatively connected at one end to said base and at the opposite end to said load-handler mounting means, said differential means including anchoring means in a fixed position relative to said base and means connected to said load-handler mounting means for pivotal movement with said load-handling means about said second pivot axis, said anchoring means anchoring said differential means at a fixed position to said base, said position being offset from said first pivot axis so that as said boom is pivoted about said first pivot axis in one direction, said differential means effects a subtractive differential movement of said load-handling means relative to said boom about said second pivot axis through an arc sufficient to maintain said load-handling means in a constant attitude relative to said base, said differential means including attitude-determining means for selectively actuating said differential means independently of pivotal movement of said boom for changing the attitude of said load-handling means relative to said base.
 13. Apparatus according to claim 1 wherein said boom includes at least two boom sections pivotal relative to one another about a third pivot axis and wherein said differential means includes idler means pivotable about said third pivot axis.
 14. Apparatus according to claim 1 wherein said differential means extends within said boom.
 15. Apparatus according to claim 1 wherein said load-handling apparatus comprises a log loader with said base comprising a mobile base, a log-lifting means pivoted to the free end of said boom, said load-handling means comprising a heel boom pivoted to the free end of said boom.
 16. Apparatus according to claim 1 including load-lifting means carried near the free end of said boom for cooperation with said load-handling means.
 17. Apparatus according to claim 16 wherein said load-handling means comprises a heel boom and said load-lifting means is pivoted to said boom for movement about said second pivot axis.
 18. Apparatus according to claim 16 wherein said load-handling means comprises a heel boom and said load-lifting means is pivoted to said heel boom. 